1. Field of the Invention
The present invention relates to a thermoplastic elastomer composition suitable for use in automobile parts. More particularly, the present invention relates to a thermoplastic elastomer composition having excellent flowability, mechanical properties and oil resistance, and capable of being lowered in hardness. Also, the present invention is concerned with a method for the preparation of the same.
2. Description of the Prior Art
It is known in the art that rubber cannot be recycled and is of low productivity. Extensive attempts have been made to develop substitutes capable of overcome the drawbacks of rubber, leading to the development of thermoplastic elastomer (TPE).
TPE, a recyclable rubber material showing properties of both plastic and rubber, is now widely used as a substitute of rubber in the field of automobile parts industries. Also, TPE is rising as a promising alternative for soft PVC because PVC causes environmental problems.
Commercially available TPE includes styrenic thermoplastic elastomer (STPE) and olefinic thermoplastic vulcanicate (TPV) made from a blend of polypropylene (PP)/ethylene propylene diene terpolymer (EPDM) by dynamic vulcanization.
What is required for rubber materials for automobile parts is superiority in compression set, mechanical physical properties, processability, heat resistance at high temperature, and oil resistance. STPE is applied mainly for miscellaneous goods but not used as materials for main parts due to its poor chemical resistance, oil resistance and compression set at high temperature. As for TPV, its applications are found in main automobile parts. However, TPV is very difficult to prepare into articles of low hardness. Practically, TPV is mainly used to make functional parts having a hardness of 55A or greater. Difficulty in preparing articles of low hardness with TPV is due to the fact that PP which itself is of high hardness forms a continuous matrix (main phase) with EPDM rubber dispersed as domains therein.
Extensive studies have been performed to overcome the drawbacks of TPV. For example, U.S. Pat. Nos. 3,662,206, 3,758,643 and 3,806,558 and EP 0 338 880 teach that a blend of polyolefin resin and EPDM rubber is treated with peroxide to partially vulcanize the EPDM rubber. The compositions thus obtained are re-processable and can be molded into articles having good surface properties. However, the compositions are limitedly used for automobile parts because of their high compression set and poor oil resistance.
U.S. Pat. No. 4,311,628 discloses a thermoplastic elastomer composition comprising a blend of polyolefin resin and EPDM rubber in which the rubber is fully or completely cured with phenolic curative in the presence of a curing activator. According to the '628 patent, the aforesaid composition shows excellent compression set and mechanical properties, and considerably improves oil resistance at high temperature. However, improved as it is, the oil resistance (volume change) at high temperature of the composition is still insufficient. This composition is also difficult to mold to complicated shapes due to its poor flowability. Further, because it is almost impossible according to the patent to produce compositions showing a hardness of 45A or lower as well as having a mechanical strength beyond a certain level, the composition of the patent is limitedly applied for automobile parts of hardness 55A or higher.